Motor vehicle brake systems inhibit rotation of the wheels of the vehicle to decelerate and stop the vehicles. Vehicle brake systems typically include a hydraulic brake system having disc brakes and/or drum brakes. The disc brakes include a brake pad that engages a rotor interconnected to the tire of the vehicle to decelerate the vehicle. The drum brakes include brake shoes that engage a drum interconnected to the tire of the vehicle to decelerate the vehicle.
Under light braking pressures (i.e., used to control the speed of the vehicle), brake pads may, however, only make partial contact or apply uneven force to the rotor surfaces due to circumferential variation in the disc thickness of the disc brake rotor. Circumferential variation in the disc thickness occurs as the disc brake rotor wears, leading to unstable frictional forces between the rotor and the brake pads. This unstable behavior of the disc brake rotor/brake pad friction pair may produce high dynamic contact force, which can, for example, excite strong vibration of the brake pads and generate a corresponding variation in the torque output of the disc brake system. Similarly, brake shoes may unevenly contact the brake drum due to the shape of the drum (e.g., less cylindrical).
Brake judder or brake torque variation “BTV” in vehicles having a hydraulic brake system is due to a chain of events, at the beginning of which there is unequal wear of the brake disks which leads to thickness variation of the brake disk (disc thickness variation (DTV)) or insufficient cylindricity of the drum in drum brakes. The forces created by the brake torque variation are transmitted to the tire of the vehicle and cause the tires of the vehicle to vibrate in the longitudinal direction of the tire. This vibration is thereafter transmitted to the brake system and chassis of the vehicle and results in brake roughness. Brake roughness is defined as the unexpected vibration that the driver of the vehicle feels through the steering wheel, brake pedal and seat track. The vibration associated with brake roughness can be transmitted to the driver of the vehicle, causing the driver to feel the vibrations.
In order to prevent strong vibration of the brake system, coupled brake actuation systems (i.e., brake actuation systems that are directly connected to a brake pedal of the motor vehicle) regulate the response time and pressure control of the brake pads as they are moved towards one another and into contact with the two friction surfaces to suppress rotor resonant vibration. For example, in a coupled brake actuation system, a pushrod controlled by the brake pedal may regulate the flow of hydraulic fluid from a brake line to the brake pads by exerting force on a piston of a master cylinder that pushes the hydraulic fluid from a reservoir to a pressure chamber. This increases the pressure of the coupled brake actuation system and forces hydraulic fluid through the brake lines and towards caliper pistons that act on the set of calipers and, consequently, cause the brake pads to apply force to the disc brake rotor. Increases and decreases in the pressure of the hydraulic fluid being passed through the calipers causes the brake pedal to feel extremely hard or soft to a driver of the motor vehicle.
It may, therefore, be advantageous to provide a disc brake system configured as a de-coupled electro-mechanical brake actuation system that regulates the pressure control and response time of the brake pads (independent from the brake pedal) to suppress vibration caused by circumferential variation in the disc thickness of the disc brake rotor or by variation in the cylindicity of the drum.